Addition of (A) blocking agent(s) in a ceramic membrane for blocking crystalline growth of grains during atmospheric sintering

ABSTRACT

A method of preparing a composite includes the following steps. A powder blend is sintering while an oxygen partial pressure (pO 2 ) of a gaseous atmosphere surrounding the powder blend is controlled. Before the sintering, a shape is formed from the powder blend. After the forming and before the sintering, binder is removed from the powder blend. The powder blend comprises binder, a mixed electronic/oxygen O 2−  anionic conducting compound (C 1 ) and a compound (C 2 ) chosen from MgO and BaTiO 3 . The resultant composite comprises at least 75 vol % of compound (C 1 ), from 0.01 to 25 vol % of compound (C 2 ), and from 0 vol % to 2.5 vol % of compound (C 3 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/577,867, filed May 8, 2007, now U.S. Pat. No. 7,955,526 which is a371 of International PCT Application PCT/FR04/82851, filed Nov. 5, 2004,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The subject of the present invention is a mixed electronic/O²⁻ anionicconducting composite, its method of preparation and its use as solidelectrolyte in a catalytic membrane reactor intended in particular forthe production of syngas by the reforming of methane or natural gas.

Catalytic membrane reactors, called CMRs hereafter, produced fromceramics, are used for separating oxygen from air by diffusion of thisoxygen in ionic form through the ceramic and the chemical reaction ofthe latter with natural gas (mainly methane) on catalytic sites (Ni ornoble metal particles) deposited on the surface of the membrane.Conversion of syngas into liquid fuel by the GTL (Gas To Liquid) processrequires an H₂/CO molar ratio of 2. This ratio of 2 can be obtaineddirectly by a process involving a CMR.

However, ceramics are brittle in behavior and have mechanical propertiesthat depend directly on the microstructure (shape and size of thegrains, secondary phases, porosity). All other things being equal, themechanical strength of a ceramic used as a CMR increases when the grainsize of which the ceramic is composed decreases. The grain size mayincrease during operation at temperature and limit the lifetime of thesystem. Various publications disclose solutions intended to improve thislifetime.

U.S. Pat. No. 5,306,411 and U.S. Pat. No. 5,478,444 disclose compositesconsisting of a mixture of an electronic conducting material and anionic conducting material, thus constituting a solid electrolyte ofmixed conductivity.

U.S. Pat. No. 5,911,860 discloses a material essentially consisting of amixed or ionic conductor and of a constituent with a chemical naturedifferent from the mixed conductor, preferably a metal with a content of0 to 20 wt %. This publication highlights the need for a second phase inorder to limit cracking of the material during sintering and thus toincrease its mechanical properties while improving its catalyticefficiency.

U.S. Pat. No. 6,187,157 discloses multiphase systems comprising a mixedionic/electronic conducting phase or just an ionic conducting phase anda second electronic conducting phase so as to improve the catalyticproperties of the material. The secondary phase is generally metallicand occupies 13% of the volume of the material.

U.S. Pat. No. 6,332,964 discloses either a dense membrane or a poroussupport consisting of a phase comprising a mixed metal oxide of ionicconductivity of the MCeO_(x),MZrO_(x) type (M: family of lanthanides) ormixed conductivity (LaSrGaMgO_(x)) and of a second phase having anelectronic conductivity (metal, metal alloy or mixed oxide of theLaSrMO_(x) type where M=transition element), said second phase beingbetween 1 and 30 vol % of the matrix. United States patent applicationUS 2002/0022568 discloses a material of formulaLn_(1−x)Sr_(y)Ca_(x−y)MO_(3−δ) (Ln: family of lanthanides and yttrium,or a mixture of the two; M: transition metal or mixture of transitionmetals) having a high mixed conductivity, a low thermal expansioncoefficient and improved mechanical properties. U.S. Pat. No. 6,471,921discloses a mixed conducting multiphase material whose secondary phasesdo not participate significantly in the conduction but do increase themechanical properties of the material. The secondary phases result froma departure from stoichiometric mixing of the precursors used tosynthesize the mixed conductor and are therefore by-products of thereaction. The content of secondary phases is between 0.1 and 20 wt %.The main material is a brown-millerite phase of structureA_(x)A′_(x′)A″_((2−x−x′))B_(y)B′_(y′)B″_((2−y−y′))O_(5+z) and thesecondary phases have compositions (A,A′)₂(B,B′)O₄, A′₂(B,B′)O₄,(A,A′)(B,B′)₂O₄, . . . etc. All these secondary phases result from thereaction for synthesizing the material. They are not added before theforming of the material.

The Applicant has sought to develop a composite that has a fine uniformstructure with grains having a size close to one micron, therebyguaranteeing high and lasting mechanical properties.

BRIEF SUMMARY OF THE INVENTION

This is why, according to a first aspect, one subject of the inventionis a composite (m) comprising:

-   -   at least 75 vol % of a mixed electronic/oxygen O²⁻ anionic        conducting compound (C₁) chosen from doped ceramic oxides which,        at the use temperature, are in the form of a crystal lattice        having oxide ion vacancies and more particularly in the form of        a cubic phase, fluorite phase, aurivillius-type perovskite        phase, brown-millerite phase or pyrochlore phase; and    -   from 0.01 to 25 vol % of a compound (C₂), different from        compound (C₁), chosen from ceramics of oxide type, ceramics of        nonoxide type, metals, metal alloys or mixtures of these various        types of materials; and from 0 vol % to 2.5 vol % of a compound        (C₃) produced from at least one chemical reaction represented by        the equation:        xF _(C1) +yF _(C2) →zF _(C3),        in which equation F_(C1), F_(C2) and F_(C3) represent the        respective crude formulae of compounds C₁, C₂ and C₃ and x, y        and z represent rational numbers greater than or equal to 0.

In the presentation that follows, compound (C₂) is often referred to asa blocking agent, in that its presence in the composite according to thepresent invention inhibits crystalline growth of the grains of compound(C₁) during one or more of the steps of the method for its manufacture.The grains of the blocking agent preferably have a shape that can liewithin a sphere with a diameter ranging from 0.1 μm to 5 μm, andpreferably less than 1 μm, whether the grains are of equiaxed shape orare acicular grains, with a length of 5 μm or less.

The expression “compound (C₁) or (C₂)” means that the composite asdefined above may comprise:

-   -   either a compound (C₁) mixed with a single compound (C₂);    -   or a combination of several compounds (C₁) mixed with a single        compound (C₂);    -   or a compound (C₁) mixed with a combination of several compounds        (C₂);    -   or a combination of several compounds (C₁) mixed with a        combination of several compounds (C₂).

The term “volume fraction” is understood to mean, in the definition ofthe composite according to the present invention, the volume fraction inthe final composite.

According to a first preferred embodiment of the present invention, thevolume fraction of compound (C₃) in the composite does not exceed 1.5%and more particularly it does not exceed 0.5% by volume.

According to one particular aspect of this preferred embodiment,compound (C₂) is essentially chemically inert with respect to compound(C₁) over the temperature range lying between room temperature and thesintering temperature, this range including the operating temperature,and the volume fraction of compound (C₃) in the composite tends toward0.

According to a second preferred aspect of the present invention, thevolume fraction of compound (C₂) is not less than 0.1% but does notexceed 10%, and more particularly the volume fraction of compound (C₂)does not exceed 5% but is not less than 1%.

In the composite as defined above, compound (C₂) is mainly chosen:

-   -   either from oxide-type ceramics, such as for example magnesium        oxide (MgO), calcium oxide (CaO), aluminum oxide (Al₂O₃),        zirconium oxide (ZrO₂), titanium oxide (TiO₂), mixed strontium        aluminum oxides SrAl₂O₄ or Sr₃Al₂O₆, mixed oxides of perovskite        structure, such as for example BaTiO₃ or CaTiO₃ or, more        particularly ones having a structure ABO_(3−δ), such as for        example        La_(0.5)Sr_(0.5)Fe_(0.9)Ti_(0.1)O_(3−δ)        or        La_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ;)    -   or from materials of the nonoxide (carbide, nitride, boride)        type such as for example silicon carbide (SiC) or boron nitride        (BN);    -   or from metals such as for example nickel, platinum, palladium        or rhodium.

According to a first particular aspect of the composite as definedabove, compound (C₁) is chosen from oxides of formula (I):(R_(a)O_(b))_(1−x)(R_(c)O_(d))_(x)  (I),in which:

R_(a) represents at least one trivalent or tetravalent atom mainlychosen from bismuth (Bi), cerium (Ce), zirconium (Zr), thorium (Th),gallium (Ga) and hafnium (Hf), and a and b are such that the structureR_(a)O_(b) is electrically neutral;

R_(c) represents at least one divalent or trivalent atom chosen mainlyfrom magnesium (Mg), calcium (Ca), barium (Ba), strontium (Sr),gadolinium (Gd), scandium (Sc), ytterbium (Yb), yttrium (Y), samarium(Sm), erbium (Er), indium (In), niobium (Nb) and lanthanum (La), and cand d are such that the structure R_(c)O_(d) is electrically neutral;and

in which x is generally between 0.05 and 0.30 and more particularlybetween 0.075 and 0.15.

Examples of oxides of formula (I) include cerium stabilized oxides,gallates and zirconias.

According to this first particular aspect, compound (C₁) is preferablychosen from stabilized zirconias of formula (Ia):(ZrO₂)_(1−x)(Y₂O₃)_(x)  (Ia),in which x is between 0.05 and 0.15.

According to a second particular aspect of the composite as definedabove, compound (C₁) is chosen from perovskites of formula (II):[Ma_(1−x−u)Ma′_(x)Ma″_(u)][Mb_(1−y−v)Mb′_(y)Mb″_(v)]O_(3−w)  (II)in which:

-   -   Ma represents an atom chosen from scandium, yttrium, or from the        families of lanthanides, actinides or alkaline-earth metals;    -   Ma′, which is different from Ma, represents an atom chosen from        scandium, yttrium or from the families of lanthanides, actinides        or alkaline-earth metals;    -   Ma″, which is different from Ma and Ma′, represents an atom        chosen from aluminum (Al), gallium (Ga), indium (In), thallium        (Tl) or from the family of alkaline-earth metals;    -   Mb represents an atom chosen from transition metals;    -   Mb′, which is different from Mb, represents an atom chosen from        transition metals, aluminum (Al), indium (In), gallium (Ga),        germanium (Ge), antimony (Sb), bismuth (Bi), tin (Sn), lead (Pb)        and titanium (Ti);    -   Mb″, which is different from Mb and Mb′, represents an atom        chosen from transition metals, alkaline-earth metals, aluminum        (Al), indium (In), gallium (Ga), germanium (Ge), antimony (Sb),        bismuth (Bi), tin (Sn), lead (Pb) and titanium (Ti);    -   0<x≦0.5;    -   0≦u≦0.5;    -   (x+u)≦0.5;    -   0≦y≦0.9;    -   0≦v≦0.9;    -   0≦(y+v)≦0.9; and    -   w is such that the structure in question is electrically        neutral.

According to this second particular aspect, compound (C₁) is moreparticularly chosen from compounds of formula (II) in which u is equalto zero or else from compounds of formula (II) in which u is differentfrom zero or else from compounds of formula (II) in which the sum (y+v)is equal to zero or else from compounds of formula (II) in which the sum(y+v) is different from zero.

In formula (II) as defined above, Ma is more particularly chosen fromLa, Ce, Y, Gd, Mg, Ca, Sr and Ba. In this case, compound (C₁) ispreferably a compound of formula (IIa):La_((1−x−u))Ma′_(x)Ma″_(u)Mb_((1−y−v))Mb′_(y)Mb″_(v)O_(3−δ)  (IIa),corresponding to formula (II), in which Ma represents a lanthanum atom.

In formula (II) as defined above, Ma′ is more particularly chosen fromLa, Ce, Y, Gd, Mg, Ca, Sr and Ba. In this case, compound (C₁) ispreferably a compound of formula (IIb):Ma_((1−x−u))Sr_(x)Ma″_(u)Mb_((1−y−v))Mb′_(y)Mb″_(v)O_(3−δ)  (IIb),corresponding to formula (II) in which Ma″ represents a strontium atom.

In formula (II) as defined above, Mb is more particularly chosen fromFe, Cr, Mn, Co, Ni and Ti. In this case, compound (C₁) is preferably acompound of formula (IIc):Ma_((1−x−u))Ma′_(x)Ma″_(u)Fe_((1−y−v))Mb′_(y)Mb″_(v)O_(3−δ)  (IIc),corresponding to formula (II) in which Mb represents an iron atom.

In formula (II) as defined above, Mb′ is more particularly chosen fromCo, Ni, Ti and Ga while Mb″ is more particularly chosen from Ti and Ga.

In this case, compound (C₁) is preferably a compound of formula (IId):La_((1−x))Sr_(x)Fe_((1−v))Mb″_(v)O_(3−δ)  (IId),corresponding to formula (II) in which u=0, y=0, Mb represents an ironatom, Ma represents a lanthanum atom and Ma′ represents a strontiumatom. In formula (II) as defined above, Ma″ is more particularly chosenfrom Ba, Ca, Al and Ga. In the composite according to the presentinvention, compound (C₁) is more particularly a compound of formula:La_((1−x−u))Sr_(x)Al_(u)Fe_((1−v))Ti_(v)O_(3−δ),La_((1−x−u))Sr_(x)Al_(u)Fe_((1−v))Ga_(v)O_(3−δ),La_((1−x))Sr_(x)Fe_((1−v))Ti_(v)O_(3−δ),La_((1−x))Sr_(x)Ti_((1−v))Fe_(v)O_(3−δ),La_((1−x))Sr_(x)Fe_((1−v))Ga_(v)O_(3−δ) or La_((1−x))Sr_(x)FeO_(3−δ) andmore particularly one of the following compounds:La_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ), orLa_(0.5)Sr_(0.5)Fe_(0.9)Ti_(0.1)O_(3−δ).

Among the compounds with a crystal structure of the perovskite type offormula (II) that are preferred to be used in the composite according tothe present invention are those of formula (II′):Ma^((a)) _((1−x−u))Ma′^((a−1)) _(x)Ma″^((a″)) _(u)Mb^((b))_((1−s−y−v))Mb^((b+1)) _(s)Mb′^((b+β)) _(y)Mb″^((b″))_(v)O_(3−δ)  (II′),in which formula (II′):

a, a−1, a″, b, (b+1), (b+β) and b″ are integers representing therespective valences of the Ma, Ma′, Ma″, Mb, Mb′ and Mb″ atoms; and a,a″, b, b″, β, x, y, s, u, v and δ are such that the electricalneutrality of the crystal lattice is preserved:

-   -   a>1;    -   a″, b and b″ are greater than zero;    -   −2≦β≦2;    -   a+b=6;    -   0<s<x;    -   0<x≦0.5;    -   0≦u≦0.5;    -   (x+u)≦0.5;    -   0≦y≦0.9;    -   0≦v≦0.9;    -   0≦(y+v+s)≦0.9;    -   [u(a″−a)+v(b″−b)−x+s+βy+2δ]=0; and    -   δ_(min)<δ<δ_(max) where    -   δ_(min)=[u(a−a″)+v(b−b″)−βy]/2 and    -   δ_(max)=[u(a−a″)+v(b−b″)−βy+x]/2        and Ma, Ma′, Ma″, Mb, Mb′ and Mb″ are as defined above, Mb        representing an atom chosen from transition metals capable of        existing in several possible valences.

According to a third particular aspect of the material as defined above,compound (C₁) is chosen from materials of the brown-millerite family offormula (III):[Mc_(2−x)Mc′_(x)][Md_(2−y)Md′_(y)]O_(6−w)  (III)in which,

-   -   Mc represents an atom chosen from scandium, yttrium or from the        families of lanthanides, actinides and alkaline-earth metals;    -   Mc′, which is different from Mc, represents an atom chosen from        scandium, yttrium or from the families of lanthanides, actinides        and alkaline-earth metals;    -   Md represents an atom chosen from transition metals; and    -   Md′, which is different from Md, represents an atom chosen from        transition metals, aluminum (Al), indium (In), gallium (Ga),        germanium (Ge), antimony (Sb), bismuth (Bi), tin (Sn), lead (Pb)        and titanium (Ti); and x and y are greater than or equal to 0        and less than or equal to 2 and w is such that the structure in        question is electrically neutral.

According to this third particular aspect of the material according tothe present invention, compound (C₁) is more particularly of formula(IIIa):[Mc_(2−x)La_(x)][Md_(2−y)Fe_(y)]O_(6−w)  (IIIa),a compound of formula (IIIb):[Sr_(2−x)La_(x)][Ga_(2−y)Md′_(y)]O_(6−w)  (IIIb)and more particularly a compound of formula (IIIc):[Sr_(2−x)La_(x)][Ga_(2−y)Fe_(y)]O_(6−w)  (IIIc),such as for example compounds of formula:Sr_(1.4)La_(0.6)GaFeO_(5.3); Sr_(1.6)La_(0.4)Ga_(1.2)Fe_(0.8)O_(5.3);Sr_(1.6)La_(0.4)GaFeO_(5.2);Sr_(1.6)La_(0.4)Ga_(0.8)Fe_(1.2)O_(5.2);Sr_(1.6)La_(0.4)Ga_(0.6)Fe_(1.4)O_(5.2);Sr_(1.6)La_(0.4)Ga_(0.4)Fe_(1.6)O_(5.2);Sr_(1.6)La_(0.4)Ga_(0.2)Fe_(1.8)O_(5.2); Sr_(1.6)La_(0.4)Fe₂O_(5.2);Sr_(1.7)La_(0.3)GaFeO_(5.15);Sr_(1.7)La_(0.3)Ga_(0.8)Fe_(1.2)O_(5.15);Sr_(1.7)La_(0.3)Ga_(0.6)Fe_(1.4)O_(5.15);Sr_(1.7)La_(0.3)Ga_(0.4)Fe_(1.6)O_(5.15);Sr_(1.7)La_(0.3)Ga_(0.2)Fe_(1.8)O_(5.15); Sr_(1.8)La_(0.2)GaFeO_(5.1);Sr_(1.8)La_(0.2)Ga_(0.4)Fe_(1.6)O_(5.1);orSr_(1.8)La_(0.2)Ga_(0.2)Fe_(1.8)O_(5.1).

According to one more particular aspect of the present invention, thesubject thereof is a composite as defined above in which compound (C₁)is chosen from compounds of formula:La_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ)orLa_(0.5)Sr_(0.5)Fe_(0.9)Ti_(0.1)O_(3−δ),and compound (C₂) is chosen from magnesium oxide (MgO), aluminum oxide(Al₂O₃), mixed strontium aluminum oxide Sr₃Al₂O₆ and mixed bariumtitanium oxide (BaTiO₃).

According to the latter particular aspect, the composite which comprisesbetween 2 and 10 vol % magnesium oxide (MgO) and between 90 and 98 vol %La_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) is preferred.

According to a second aspect, another subject of the invention is amethod of preparing the composite as defined above, characterized inthat it includes at least one step of sintering a powder blend ofcompound (C₁) and compound (C₂), while controlling the oxygen partialpressure (pO₂) of the gaseous atmosphere surrounding the reactionmixture.

In the method as defined above, the sintering temperature of thecomposite is between 800° C. and 1500° C., preferably between 1000° C.and 1350° C.

The sintering includes two simultaneous phenomena that are generally incompetition, namely densification of the material by elimination of theporosity and grain growth. If the densification of the material has tobe maximized for its use as a mixed conductor, crystal growth can bedetrimental to its mechanical properties. The sintering step musttherefore be adapted in order to result in densification of the partwhile minimizing grain growth. However, it is often difficult for thesetwo conditions, depending on the nature of the materials used, or of theimposed sintering conditions, to be met. The presence of a suitableamount of compound (C₂) in the mixed conductor ensures satisfactorydensification while limiting, or even preventing, crystalline growth ofthe conductor (C₁).

The method as defined above is more particularly used in such a way thatthe sintering step is carried out in a gaseous atmosphere having anoxygen partial pressure of 0.1 Pa or less.

According to another particular aspect, the method as defined above ischaracterized in that the powder blend of compound (C1) and compound(C2) undergoes, before the sintering step, a forming step followed bybinder removal.

According to another aspect, another subject of the invention is the useof the composite as defined above, as a mixed conducting composite for acatalytic membrane reactor, intended to be used for the synthesis ofsyngas by catalytic oxidation of methane or natural gas and/or as mixedconducting composite for a ceramic membrane intended to be used forseparating oxygen from air.

The final subject of the invention is a method for inhibiting and/orcontrolling the crystal growth of the grains of mixed electronic/oxideionic conducting compounds during the sintering step in the preparationof a catalytic membrane reactor, characterized in that it includes aprior step of blending 75 to 99.99 vol % of mixed conductor (C₁) with0.01 to 25 vol % of compound (C₂).

According to a preferred aspect of the method as defined above, thisincludes a prior step of blending 90 to 98 vol %La_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) with 2 to 10 vol % magnesiumoxide (MgO).

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a further understanding of the nature and objects for the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 shows two images obtained by scanning electron microscopy withtwo different magnifications (FIG. 1 a: ×8000 and FIG. 1 b: ×10000).

FIG. 2 shows maps of the constituent elements of the membrane, obtainedby EDS analysis.

FIG. 3 shows by X-ray diffraction the fact that the MgO (40 vol%)/La_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) (60 vol %) blend does notresult, after being calcined at 1200° C. for a few hours in nitrogen, inany new compounds.

FIG. 4 shows by X-ray diffraction that the BaTiO₃ (40 vol%)/La_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) (60 vol %) blend does notresult, after being calcined in nitrogen at 1200° C. for a few hours, inany new compounds.

FIG. 5 is a secondary-electron SEM micrograph of the compositecontaining no blocking agent (magnification: ×3000; grain size between 2and 10 μm).

FIG. 6 is a secondary-electron SEM micrograph of theLa_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) composite containing 5 vol %magnesium oxide as blocking agent (magnification: ×20000; grain sizebetween 0.1 and 1 μm).

FIG. 7 is a secondary-electron SEM micrograph of theLa_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) composite containing 5 vol %magnesium oxide as blocking agent (magnification: ×50000; grain sizebetween 0.2 and 1.6 μm).

FIG. 8 is device used to carry out permeation measurements including atube furnace and gas feeds and analyzers (gas chromatography—YSZ-basedoxygen probe).

FIG. 9 shows curves of the variation in oxygen flux as a function oftemperature for each of the composites.

FIG. 10 shows the impact of the membrane microstructure on theactivation energy of the oxygen permeation flux, which energy decreaseswhen the proportion of magnesium oxide increases.

DETAILED DESCRIPTION OF THE INVENTION

Manufacture of the Multiphase Composite

The blocking agent is generally obtained from a commercial powder ofhigh purity or from a powder blend. It may also be synthesized fromoxide and/or nitrate and/or carbonate precursors blended and homogenizedin a suitable manner. This precursor blend is then calcined at hightemperature, between 800° C. and 1400° C., in order to react and formthe desired composite or composites, these being checked by X-raydiffraction. If necessary, the precursor powder is milled, preferably byattrition milling, in order to tighten the particle size distributionand reduce the grain size, for example to 0.5 μm. The steps of formingthe composite, consisting of the uniform blend of particles (C₂) in thematrix (C₁), and of binder removal are identical to those for only themixed (C₁) conductor.

The high-temperature heat treatment is generally adapted to the presenceof the blocking agent, which facilitates sintering.

Example 1 MgO (5 vol %)/La_(0.5)Sr_(0.5)Fe_(0.9)Ti_(0.1)O_(3−δ) (95 vol%) Ceramic Membrane

The example presented is a blend according to the protocol describedabove, consisting of 5 vol % magnesia (MgO) (compound C₂) and 95 vol %of the ceramic La_(0.5)Sr_(0.5)Fe_(0.9)Ti_(0.1)O_(3−δ) (compound C₁).The sintering was carried out in nitrogen at 1150° C. for 1.5 h.

FIG. 1 shows two images obtained by scanning electron microscopy withtwo different magnifications (FIG. 1 a: ×8000 and FIG. 1 b: ×10000).These images show that the MgO grains are distributed uniformly withinthe matrix, and have a size of less than 1 μm. TheLa_(0.5)Sr_(0.5)Fe_(0.9)Ti_(0.1)O_(3−δ) grains are all smaller than 2μm.

An La_(0.5)Sr_(0.5)Fe_(0.9)Ti_(0.1)O_(3−δ) membrane containing no MgOand sintered under the same conditions (in nitrogen at 1150° C. for onehour and a half) had grain sizes of between 2 and 3 μm.

FIG. 2 shows maps of the constituent elements of the membrane, obtainedby EDS analysis. It may be seen that all the elements are uniformlydistributed. These maps clearly demonstrate the chemical nonreactivityof the MgO blocking agent with respect toLa_(0.5)Sr_(0.5)Fe_(0.9)Ti_(0.1)O_(3−δ).

Example 2 MgO (5 vol %)/La_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) (95 vol%) Ceramic Membrane

The example presented is a blend according to the protocol describedabove, consisting of 5 vol % magnesia (MgO) (compound C₂) and 95% of theceramic La_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) (abbreviated in thefigures to LSFG; compound C₁). The sintering was carried out in nitrogenat 1235° C. for two hours.

FIG. 3 shows by X-ray diffraction the fact that the MgO (40 vol%)/La_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) (60 vol %) blend does notresult, after being calcined at 1200° C. for a few hours in nitrogen, inany new compounds. The X-ray diffraction diagram demonstrates that thereis no chemical reactivity between the MgO blocking agent and theLa_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) ceramic matrix.

FIG. 4 shows by X-ray diffraction that the BaTiO₃ (40 vol%)/La_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) (60 vol %) blend does notresult, after being calcined in nitrogen at 1200° C. for a few hours, inany new compounds. The X-ray diffraction diagram demonstrates that thereis no chemical reactivity between the BaTiO₃ blocking agent and theLa_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) ceramic matrix.

FIG. 5 is a secondary-electron SEM micrograph of the compositecontaining no blocking agent (magnification: ×3000; grain size between 2and 10 μm). The sintering step was carried out in nitrogen for 2 hoursat 1235° C. (composite of the prior art).

FIG. 6 is a secondary-electron SEM micrograph of theLa_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) composite containing 5 vol %magnesium oxide as blocking agent (magnification: ×20000; grain sizebetween 0.1 and 1 μm). The sintering step was carried out in nitrogenfor 2 hours at 1235° C.

FIG. 7 is a secondary-electron SEM micrograph of theLa_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) composite containing 5 vol %magnesium oxide as blocking agent (magnification: ×50000; grain sizebetween 0.2 and 1.6 μm). The sintering step was carried out in nitrogenfor 2 hours at 1300° C.

Example 3 Influence of the Presence of Magnesium Oxide (MgO) inLa_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ) (LSFG) on the OxygenPermeability of the Membrane

a) Preparation of the Specimens

LSFG and MgO powders were weighed and blended in different proportionsby volume in order to obtain compositions comprising (100−x) vol % LSFGand x vol % MgO, where x=0; 2; 5.

Dense membranes with a thickness of about 1 mm and an area of 3.1 cm²were prepared from these compositions (LSFG (x=0); LSFG/2M (x=2);LSFG/5M (x=5)) using the tape casting process described in TheEncyclopedia of Advanced Materials, Volume 4, Pergamon 1994, Cambridge,2763-2767 by T. Chartier, and in which the binder removal step wascarried out with a slow heating rate and the sintering step was carriedout between 1250° C. and 1350° C. for 2 h in a 90% nitrogen/10% oxygenatmosphere. The permeation measurements were carried out with the deviceshown in FIG. 8, consisting of a tube furnace and gas feeds andanalyzers (gas chromatography—YSZ-based oxygen probe).

The dense membranes of pure (LSFG) phase and composite (LSFG/2M;LSFG/5M) phase were deposited at the top of an alumina tube, sealingbetween the inside and outside of the tube being provided by a glassring located between the support tube and the membrane and by an aluminacap in order to hold it in place and to exert pressure from above.

The entire device was inserted into the tube furnace, which was heatedup to the glass transition temperature of the glass ring.

Before sealing, the membranes were subjected to a stream of argon overtheir external surface and a stream of recombined air (79% N₂/21% O₂) ontheir internal face with flow rates of 200 ml (STP)/min. The gasesexiting the device were analyzed using a chromatograph in order tovalidate the 100% selectivity with respect to oxygen and using an oxygenprobe to determine the oxygen permeation flux through each of thecomposites.

b) Results

FIG. 9 shows curves of the variation in oxygen flux as a function oftemperature for each of the composites. It demonstrates themultiplicative factor (MF) caused by the oxygen flux thanks to thepresence of blocking agents (at 950° C., MF=4 in the case of LSFG/2Mcompared with LSFG, and MF=6 in the case of LSFG/5M compared with LSFG).

FIG. 10 shows the impact of the membrane microstructure on theactivation energy of the oxygen permeation flux, which energy decreaseswhen the proportion of magnesium oxide increases.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

1. A method of preparing a composite, comprising the steps of: sintering a powder blend while controlling an oxygen partial pressure (pO₂) of a gaseous atmosphere surrounding the powder blend; before said step of sintering, forming a shape from the powder blend, wherein: the powder blend comprises binder, a mixed electronic/oxygen O²⁻ anionic conducting compound (C₁) and a compound (C₂) chosen from MgO and BaTiO₃; the resultant composite comprises at least 75 vol % of compound (C₁), from 0.01 to 25 vol % of compound (C₂), and from 0 vol % to 2.5 vol % of compound (C₃), compound (C₃) produced from at least one chemical reaction represented by the equation: xF_(C1)+yF_(C2)→zF_(C3), in which equation F_(C1), F_(C2) and F_(C3) represent the respective crude formulae of compounds (C₁), (C₂) and (C₃) and x, y and z represent rational numbers greater than or equal to 0; compound (C₁) is chosen from perovskite oxides of formula (II): [Ma_(1−x−u)Ma′_(x)Ma″_(u)][Mb_(1−y−v)Mb′_(y)Mb″_(v)]O_(3−w)  (II) wherein: Ma represents an atom chosen from scandium, yttrium, or from the families of lanthanides, actinides or alkaline-earth metals; Ma′, which is different from Ma, represents an atom chosen from scandium, yttrium or from the families of lanthanides, actinides or alkaline-earth metals; Ma″, which is different from Ma and Ma′, represents an atom chosen from aluminum (Al), gallium (Ga), indium (In), thallium (TI) or from the family of alkaline-earth metals; Mb represents an atom chosen from transition metals; Mb′, which is different from Mb, represents an atom chosen from transition metals, aluminum (Al), indium (In), gallium (Ga), germanium (Ge), antimony (Sb), bismuth (Bi), tin (Sn), lead (Pb) and titanium (Ti); Mb″, which is different from Mb and Mb′, represents an atom chosen from transition metals, alkaline-earth metals, aluminum (Al), indium (In), gallium (Ga), germanium (Ge), antimony (Sb), bismuth (Bi), tin (Sn), lead (Pb) and titanium (Ti); 0<x≦0.5; 0≦u≦0.5; (x+u)≦0.5; 0≦y≦0.9; 0≦v≦0.9; 0≦(y+v)≦0.9; and w is such that the structure in question is electrically neutral; and after said step of forming and before said step of sintering, removing the binder from the powder blend.
 2. The method of claim 1, in which the grains of compound (C₂) have an equiaxed shape with a diameter ranging from 0.1 μm to 5 μm and preferably less than 1 μm.
 3. The method of claim 1, in which the volume fraction of compound (C₃) does not exceed 1.5%.
 4. The method of claim 3, in which the volume fraction of compound (C₃) in the composite tends toward
 0. 5. The method of claim 1, in which the volume fraction of compound (C₂) is not less than 0.1% but does not exceed 10%.
 6. The method of claim 5, in which the volume fraction of compound (C₂) does not exceed 5%.
 7. The method of claim 1, in which compound (C₁) is chosen from compounds of formula (IIa): La_((1−x−u))Ma′_(x)Ma″_(u)Mb_((1−y−v))Mb′_(y)Mb″_(v)O_(3-δ)  (IIa), wherein Ma represents a lanthanum atom.
 8. The method of claim 1, in which compound (C₁) is chosen from compounds of formula (IIb): Ma_((1−x−u))Sr_(x)Ma″_(u)Mb_((1−y−v))Mb′_(y)Mb″_(v)O_(3−δ)  (IIb), wherein Ma′ represents a strontium atom.
 9. The method of claim 1, in which compound (C₁) is chosen from compounds of formula (IIc): Ma_((1−x−u))Ma′_(x)Ma″_(u)Fe_((1-y-v))Mb′_(y)Mb″_(v)O_(3−δ)  (IIc), wherein Mb represents an iron atom.
 10. The method of claim 1, in which compound (C₁) is chosen from compounds of formula (IId): La_((1−x))Sr_(x)Fe_((1−v))Mb″_(v)O_(3−δ)  (IId), wherein u=0, y=0, Mb represents an iron atom, Ma represents a lanthanum atom, and Ma′ represents a strontium atom.
 11. The method of claim 1, in which compound (C₁) is a compound of formula: La_((1−x−u))Sr_(x)Al_(u)Fe_((1−v))Ti_(v)O_(3−δ), La_((1−x−u))Sr_(x)Al_(u)Fe_((1−v))Ga_(v)O_(3−δ), La_((1−x))Sr_(x)Fe_((1−v))Ti_(v)O_(3−δ), La_((1−x))Sr_(x)Ti_((1−v))Fe_(v)O_(3−δ), La_((1−x))Sr_(x)Fe_((1−v))Ga_(v)O_(3−δ) or La_((1−x))Sr_(x)FeO_(3−δ).
 12. The method of claim 11, in which compound (C₁) is a compound of formula: La_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ), or La_(0.6)Sr_(0.6)Fe_(0.9)Ti_(0.1)O_(3−δ).
 13. The method of claim 1, wherein: compound (C₁) is chosen from those of formula (II′): Ma^((a)) _((1−x−u))Ma′^((a−1)) _(x)Ma″^((a″)) _(u)Mb^((b)) _((1−s−y−v))Mb^((b+1)) _(s)Mb′^((b+β)) _(y)Mb″^((b″)) _(v)O_(3−δ)  (II′); a, a−1, a″, b, (b+1), (b+β) and b″ are integers representing the respective valences of the Ma, Ma′, Ma″, Mb, Mb′ and Mb″ atoms; and a, a″, b, b″, β, x, y, s, u, v and δ are such that the electrical neutrality of the crystal lattice is preserved, a>1; a″, b and b″ are greater than zero; −2≦β≦2; a+b=6; 0<s<x; 0<x≦0.5; 0≦u≦0.5; (x+u)≦0.5; 0≦y≦0.9; 0≦v≦0.9; 0≦(y+v+s)≦0.9; [u(a″−a)+v(b″−b)−x+s+βy+2δ]=0; δ_(min)<δ<δ_(max); δ_(min)=[u (a−a″)+v(b−b″)−βy]/2; δ_(max)=[u (a−a″)+v(b−b″)−βy+x]/2; Ma, Ma′, Ma″, Mb, Mb′ and Mb″ are as defined above; and Mb represents an atom chosen from transition metals capable of existing in several possible valences.
 14. The method of claim 1, wherein compound (C₁) is chosen from compounds of formula: La_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ), or La_(0.5)Sr_(0.5)Fe_(0.9)Ti_(0.1)O_(3−δ).
 15. The method of claim 14, wherein: compound (C₂) is magnesium oxide (MgO); compound (C₁) is La_(0.6)Sr_(0.4)Fe_(0.9)Ga_(0.1)O_(3−δ); and the resultant composite comprises between 2 and 10 vol % of compound (C₂) and between 90 and 98 vol % of compound (C₁).
 16. The method of claim 1, in which the gaseous atmosphere surrounding the powder blend has an oxygen partial pressure of 0.1 Pa or less.
 17. The method of claim 1, in which the volume fraction of compound (C₃) does not exceed 0.5% by volume. 